The subject matter of the present invention relates to a method for individually characterizing, from the standpoint of production performance, each of the producing layers of a hydrocarbon reservoir traversed by a well.
An accurate and reliable evaluation of a layered reservoir requires an evaluation on a layer-by-layer basis, which involves that relevant parameters, such as permeability, skin factor, and average formation pressure, can be determined for each individual layer.
A first conceivable approach for analyzing individual layers is to isolate each layer by setting packers below and above the layer, and to perform pressure transient tests, involving the measurement of downhole pressure. The layer is characterized by selecting an adequate model, the selection being accomplished using a log-log plot of the pressure change vs. time and its derivative, as known in the art. But this method is less than practical as packers would have to be set and tests conducted successively for each individual layer.
An alternative approach relies on downhole measurements of pressure and flow rate by means of production logging tools. A proposal for implementing this approach has been to simultaneously measure the flow rate above and below the layer of interest, whereby the contribution of the layer to the flow would be computed by simply subtracting the flow rate measured below the layer from the flow rate measured above this layer. This in effect would provide a substitute for the isolation of a zone by packers. But this proposal has suffered from logistical and calibration difficulties that have thwarted its commercial application. A more practical testing technique, called Multilayer Transient (MLT) testing technique, is described by Shah et al, "Estimation of the Permeabilities and Skin Factors in Layered Reservoirs with Downhole Rate and Pressure Data" in SPE Formation Evaluation (September 1988) pp. 555-566. In this technique, downhole measurements of flow rate are acquired with only one flowmeter displaced from one level to another level. Flow rate measurements are thus acquired at different times. However, because fluctuations may occur in the surface flow rate, and also because the change imposed on the surface flow rate to initiate a transient is of arbitrary magnitude, it is not possible to determine the contribution of an individual layer by simply subtracting from each other the flow rates measured below and above the layer. This complicates the interpretation of test data.